Selective uniform liquid phase epitaxial growth

ABSTRACT

A PROCESS FOR PREFORMING LIQUID PHASE EPITAXIAL GROWTH OF SEMICONDUCTOR MASTERIAL, PARTICULARLY III-V COMPOUNDS, PROVIDING BOTH DISSOLUTION AND GROWTH OF EXTREMELY UNIFORM DIMESIONS OVER A LARGE AREA, AND FUTHER WHEREIN SAID EPTIAXIAL GROWTH MAY BE ACCOMPLISHED IN A SELECTIVE FASHION. THE PROCESS HAS SIGNIFICANT APPLICATION TO AN ARRAY FABRAICATION OF HIGH EFFICIENCY LIGHT EMITTING DIODES COMPOSED OF III-V COMPOUND MATERIALS SUCH AS GAAS.

March 7, 1972 A, BARNETT ETAL 3,647,518

. SELECTIVE UNIFORM LIQUID PHASE EPITAXIAL GROWTH Filed April 30, 1970 FIG.IO

W/ /VWWI INVENTORS ALLEN M. BARNETT, HAROLD A. JENSEN,

VIRGINIA E MEIKLEHAM,

t I; t I WWW TIME THEIR ATTORNEY.

3,647,578 Patented Mar. 7, 1972 3,647,578 SELECTIVE UNIFORM LIQUID PHASE EPITAXIAL GROWTH Allen M. Barnett, Schenectady, Harold A. Jensen, Liverpool, and Virginia F. Meikleham, Syracuse, N.Y., as-

signors to General Electric Company Filed Apr. 30, 1970, Ser. No. 33,353

Int. Cl. H01! 7/38 US. Cl. 148-171 16 Claims ABSTRACT OF THE DISCLOSURE The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention relates generally to the field of liquid phase epitaxial growth of semiconductor materials, and in particular of III-V compound materials.

(2) Description of the prior art Vapor phase epitaxial growth is the most common growth process presently employed for growing semi conductor materials. Briefly, a semiconductor substrate has semiconductor material grown to a surface thereof by a process wherein a mixture of the semiconductor material, and normally a selected impurity, when in the vapor state and at elevated temperatures are applied to the substrate surface. There is subsequently formed a deposit on the substrate which comprises the growth region. Vapor phase growth provides excellent impurity and geometry control. Thus, an n-type, p-type or insulating region may be grown to a semiconductor substrate with accurate thickness over relatively large areas. However, for certain applications, growing from the vapor phase is not completely satisfactory. In particular, it is found that a vapor phase epitaxial growth of GaAs for the fabrication of light emitting diodes substantially limits the light efiiciency of said diodes, and that more recently developed liquid phase processes provide an improvement in light efliciency by as much as a factor of 10.

However, liquid phase epitaxial growth processes that have been employed to date are with limitation, principally with respect to providing uniform growth over a relatively large area. One important example of such requirement is in the fabrication of an array of semiconductor devices on a single substrate. In one known process for providing epitaxial growth of GaAs from the liquid phase, the substrate wafer to be grown to is held against the bottom and at one end of a graphite crucible. A melt of Ga, GaAs plus an impurity is placed at the opposite end of the crucible. The crucible is at an inclined angle within a horizontal furnace so that the solution covers but one half of the bottom surface of. the crucible, the remaining surface holding the substrate being dry. Upon heating of the furnace so as to bring the solution up to a temperature where it is at or near saturation, the furnace is tipped and the melt flowed over the exposed surface of the GaAs substrate. Initially, the GaAs at the exposed surface of the substrate dissolves in the solution until equilibrium is established. As the temperature is then reduced below the equilibrium point, precipitation of GaAs from the solution occurs, which produces epitaxial growth upon the substrate. This process results in a relatively uneven dissolution and growth of the semiconductor due principally to the temperature gradients in the melt as it covers the wafer, and to the method of flowing the melt over the substrate which provides uneven wetting, that is, the front surface of the substrate is contacted prior and for a longer time than the rear surface.

In another process that has been employed for liquid phase epitaxial growth of semiconductor material a crucible is partially filled with a solution of a semiconductor material. The crucible is then placed in a vertical furnace and the semiconductor substrate to be grown to is positioned above the crucible in a vertical alignment. Upon heating of the solution and wafer to temperature, the wafer is submerged on edge into the solution. As in the previously described process, a dissolution will first occur and then upon the solution being cooled the GaAs will precipitate out and provide growth onto the substrate surface. Again, due to uneven temperature gradients Within the solution and the method of inserting the wafer on edge into the solution, nonuniform dissolution and growth result.

BRIEF SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a novel method of liquid phase epitaxial growth for semiconductor material which provides growth of uniform thickness over a relatively large area.

A further object of the invention is to provide a novel method of liquid phase epitaxial growth for semiconductor material wherein a selective uniform growth is performed over a given area.

Another object of the invention is to provide a novel liquid phase epitaxial growth process as above described which has particular application to III-V compounds.

Another more specific object of the invention is to provide a novel liquid phase epitaxial growth of semiconductor material wherein there is obtained a uniform dissolution and regrowth at selected areas of a GaAs substrate.

It is yet another object of the invention to provide a novel apparatus for performing a liquid phase epitaxial growth of semiconductor materials in accordance with the above described process.

In accordance with these and other objects of the invention, a uniform liquid phase epitaxial growth of a semiconductor material is accomplished by a process which basically includes preparing a molten solution containing said semiconductor material, said solution being at a temperature which is at or near a saturated condition of the semiconductor material within said solution. A semiconductor substrate having a given surface for growth is supported above said solution by a graphite plate member, said substrate being secured to the upper surface of said member with said given surface free. The substrate and plate member are heated to a temperature in the order of, but usually slightly higher than, the temperature of said molten solution. Following this, the substrate is immersed into said solution so that said plate enters first and said substrate enters second with its given surface in approximate parallel alignment with the surface of said solution, the solution flowing substantially at once over said given surface. When the substrate is immersed in said solution, a uniform thickness of the semiconductor material at said given surface initially dissolves into said solution until an equilibrium condition at the interface is reached. Upon cooling of the solution a quantity of the semiconductor material precipitates out of the solution and becomes uniformly epitaxially deposited on said given surface. After cooling and at the end of the regrowth, the substrate is removed from the solution and the apparatus is spun to remove excess solution.

In a specific process for selectively growing GaAs the solution may comprise a mixture of Ga, GaAs and a suitable impurity such as tellurium, selenium or tin for n-type conductivity growth, or zinc for p-type conductivity growth. The substrate may be oxygen doped GaAs, for which the starting temperature is preferably between 800 C. and 840 C., with a cooling rate of approximately 1 C. per minute. The cooling cycle extends to a final temperature of between 790 C. and 800 C. An SiO film deposited to a thickness of about 1500 A. to 2000 A. is found to be a suitable masking material against the prepared molten solution. By performing a uniform dissolution and regrowth of the substrate within the windows of the mask, a process yieiding a planar or nearly planar surface is accomplished.

BRIEF DESCRIPTION OF THE DRAWING The specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention. It is believed, however, that both as to its organization and method of operation, together with further objects and advantages thereof, the invention may be best understood from the description of the preferred embodiments, taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of apparatus for performing a liquid phase epitaxial growth in accordance with the invention;

FIG. 2 is a plan view of the substrate holder of the apparatus in FIG. 1;

FIG. 3 is a cross-sectional view of the substrate holder of FIG. 2 taken along the plane 33;

FIG. 4 is a cross-sectional view schematically illustrating a first-step of the growth process wherein the substrate is held above the molten solution.

FIG. 5 is a cross-sectional view schematically illustrating a second step of the process wherein the holder which Supports the substrate is brought in contact with the surface of the solution;

FIG. 6 is a cross-sectional view schematically illustrat-.

ing a third step of the process wherein the substrate is totally submerged in the solution;

FIG. 7 is a cross-sectional view schematically illustrating a fourth step of the process wherein the substrate is removed from the solution and spin dried;

FIG. 8 is a diagram illustrating a typical temperature cycle for growing GaAs material in accordance with one embodiment of the invention;

FIG. 9 is a plan view of the face of asubstrate upon which a selective growth has been made in accordance with the invention; and

FIG. 10 is a cross-sectional view of a portion of FIG. 9 taken along the plane 10-10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1 there is schematically illustrated in cross-sectional view an apparatus 1 for performing a liquid phase epitaxial growth of a semiconductor material with respect to a semiconductor substrate 2, in accordance with the process of the present invention. The substrate 2 is commonly of the same material as the grown material, but may also be of a different semiconductor material as where heterojunctions are to be formed. The present process provides both dissolution and growth of extremely uniform dimensions over a relatively large area of a given surface 2a of the substrate. The apparatus 1 includes a quartz tube 3 located within a furnace 4 which may be of conventional vertical furnacedesign with resistance heating. The quartz tube 3 has an input port 5 for introducing an inert gas during the heating and cooling cycles of the process. The gas is typically hydrogen, argon or forming gas nitrogen, 10% hydrogen). A tube cover 6 fits over the tube, having a gas exhaust port 7.' A crucible 8 of a material exhibiting good thermal conduction, typically graphite, is partially filled with a saturated or nearly saturated solution 9 of said semiconductor material, the crucible 8 standing upon a quartz spacer 10 so as to be situated within a uniformly heated zone of the furnace. A substrate holder 11 in the shape of a plate, with the substrate 2 secured to its upper surface, is positioned by a positioning rod 12 within the furnace. The holder is initially positioned above the solution as illustrated. The holder 11 is also of a material exhibiting good thermal conduction properties such as graphite. The tube cover 6 includes an opening through which the positioning rod 12 is guided during vertical movement so as to provide immersion of the substrate 2 and the holder 11- 'into the solution 9. A thermocouple 14 extends through the tube cover 6 and fits into a channel in the crucible 8 for determining t-he temperature of the crucible, and therefore the solution. The thermocouple may be moved between the illustrated lower position-and an upper position alongside the substrate and holder for measuring temperature of the substrate.

In FIG. 2 there is a plan view of one specific form of the substrate holder 11 which is seen to include two leaves 20 and 21 on the upper surface, the edges of which are undercut for providing grooves 22 within which the substrate 2 is held. A'pair of threaded arms 23 are attached to the leaves. As shown in the cross-sectional 'view of FIG. 3 taken along the plane 3 3, the upper surface of the holder 11 is slightly inclined. The angle of inclination is within a few degrees and the purpose is to permit excess solution to run off after withdrawing the substrate from the solution. It is noted that the angle of inclination is small enough so that upon immersion the substrate is in a position approximately parallel to the surface of the solution. The arms 23 are joined by a bridge 24, shown in FIG. 1, to which the positioning rod 12 firmly attaches.

There will now be considered the liquid phase epitaxial growth process of the present invention in a step by step sequence. In a preparatory procedure the solution 9 is heated within the furnace and brought up to temperature in the presence of an inert gas introduced into the input port 5. The substrate 2 and holder 11 are then introduced into the quartz tube and held there for several minutes in a cold zone until the oxygen is removed. Following this, in what may be considered a first step of the process illustrated in the cross-sectional view of FIG. 4, the substrate 2 and holder 11 are positioned above solution 9 within the heating zone of the furnace. The substrate 2 is held in this position until heated to a temperature in the order of that of the solution. Most commonly, the substrate is brought to a temperature slightly higher than that of the solution. In a second step of the process, illustrated by the cross-sectional view of FIG. 5, the substrate and holder are lowcred toa position in which the holder rests on the surface of the solution, where it is held for several seconds. This aids to smooth out the temperature gradients in this region of the. solution. In a third step of the process, the substrate and holder are plunged vertically into the solution 9 to rest on the bottom of the crucible 8. as illustrated in the cross-sectional view of FIG. 6. This immersing to provide good junction characteristics between the regrown region and the substrate proper. The dissolution continues, normally for several seconds to several minutes, until an equilibrium condition is reached at the interface wherein this region of the solution is again in a saturated condition. The extent of the dissolution and therefore the amount of material removed from the substrate surface 2a is determined by the percentage of unsaturation of the solution at the interface. This can be controlled by several factors including the starting temperature at immersion, the temperature differential between the solution and substrate at immersion, and the temperature cycling of the solution subsequent to immersion. Care must be tak n to prevent excessive dissolution from occurring.

Shortly after the substrate 2 is immersed into the solution, the temperature of thefurnace is lowered and upon falling below the temperature of equilibrium, the semiconductor material within the solution will precipitate out and become deposited on the substrate surface 2, which is the liquid phase epitaxial regrowth. Regrowth continues as the temperature of the solution continues to be reduced until a minimum point where the process is terminated. After the regrowth process is completed, the substrate is vertically withdrawn from the solution and spun to remove excess solution of the substrate surface, as illustrated in the cross-sectional view of FIG. 7.

In one specific example of the process GaAs was grown as an n-type region on a GaAs substrate. The solution included 30 grams Ga, 5 grams GaAs and 50 milligrams tellurium. The substrate was a single crystal oxygen doped GaAs wafer, having a Weight in the order of one gram, an area of about 1" square and about mils in thickness. The 111 Ga face formed the surface 2a to which growth was made.

A typical temperature cycle is illustrated in FIG. 8. Plotted is the temperature of the solution versus time beginning at t when the substrate is introduced into the hot zone of the furnace. As seen in this figure, the starting temperature for the solution is about 815 C. This temperature is maintained for about 10 minutes while the substrate is heated. Since the substrate is located in a slightly hotter region of the furnace than is the solution it will heat to a temperature about 5 C.10 C. above that of the solution. Upon immersion of the substrate at t the temperature of the solution is seen to rise slightly, about a couple of degrees, during the first minute of immersion. At t the temperature begins to fall during the time in which the furnace is cooled. The cooling rate is at approximately 1 per minute, the temperature being reduced to about 790 C. after about 30 minutes at The above illustrated temperature cycle was employed with respect to a specific substrate composition and a specific growth material for obtaining a desired dissolution and regrowth. However, the temperatures and the temperature cycle may be varied with other materials and other dissolution and regrowth requirements. For example, tin and silicon doped GaAs substrates have been processed with starting temperatures considerably higher than that indicated in FIG. 8, extending as high as 970 C. The cooling rate may extend from about .1 C. per minute to about 10 C. per minute. For precipitating out GaAs the temperature is reduced by an amount in accordance with desired growth, including to temperatures below 790 C.

By immersing the substrate into the solution in the manner indicated, an extremely uniform dissolution and regrowth are found to result. This is believed due principally to the relatively uniform temperature gradients existing laterally in the solution coupled with the even and rapid flowing over of the substrate surface upon immersion. In the fabrication of active semiconductor devices where many devices are made from a single wafer, it may be necessary that both dissolution and regrowth be of uniform thickness in order for the devices to exhibit comparable electrical characteristics. In particular, this is true for light emitting diodes of a PSIN configuration wherein a uniform dissolution provides a-uniformly thick semi-insulating region, and a uniform regrowth provides a uniformly thick N region.

In a selective liquid phase epitaxial growth process, such as has been employed in the fabrication of a monolithic light emitting diode array, a mask 30 is first applied to the growth surface of the substrate. Both dissolution and regrowth are performed at selected areas determined by the openings in the mask to which the substrate surface is exposed. The process is otherwise identical to that which has been described. In one specific embodiment employing a GaAs substrate, the mask was comprised of SiO film by a conventional RF glow discharge process to a thickness of 1500 A. to 2000 A. This thickness of the film is found to be suflicient to retain its integrity with the substrate solution and still provide good adherence to the substrate surface. Windows 31 are etched in the SiO film using conventional photolithographic techniques during preparation of the substrate. In the example under discussion, the windows were in a column and row arrangement filling a square inch, each window being 6 mils in diameter on 50 mil centers. A plan view of a substrate that has been masked in this manner and provided with a selective growth is illustrated in FIG. 9. During the specific dissolution and regrowth process being considered, dissolution at each of the selected areas was found to be about 4 mils and regrowth about 2 mils. A cross-sectional view of a portion of the substrate with the mask etched ofi, showing the dissolution and regrowth 32, is illustrated in FIG. 10. The uniformity of dissolution and regrowth over the surface of the substrate was determined to be within i3%.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A method of producing a liquid phase epitaxial growth of semiconductor material with respect to a given surface of a semiconductor substrate, comprising the steps of:

(a) preparing a molten solution of given temperature which solution contains said semiconductor material,

(b) supporting said substrate above said solution by a holder of good thermal conduction characteristics so that the body of the holder is between the substrate and bath, said substrate being heated to a temperature in the order of said given temperature, (c) immersing said substrate into said bath so that said holder enters first and said substrate enters second with the solution flowing substantially at once over said given surface upon immersion, and

(d) cooling said solution so that the contained semiconductor material precipitates out and becomes deposited on said given surface.

2. A method as in claim 1 wherein said substrate enters said solution with said given surface in an alignment that is within a few degrees of parallel to the surface of said solution.

3. A method as in claim 1 in which the substrate is withdrawn from the solution after cooling and rapidly spun to remove excess solution on said given surface.

4. A method as in claim 1 in which the body of said holder is in the shape of a plate and said substrate is secured to the top surface of said plate with said given surface free.

5. A method as in claim 1 in which said substrate is composed of the same semiconductor material as is contained in said solution.

6. A method as in claim 5 in which said given temperature approximately corresponds to a saturated condition of the semiconductor material in said solution, and said substrate is "heated to a temperature slightly higher than said given temperature so that at said given surface a portion of the substrate is dissolved upon first becoming immersed in said solution prior to cooling.

7. A method as in claim 6 in which said solution includes a mixture of GaAs, Ga and an impurity and said substrate is a single crystal GaAs wafer having the III Ga face as said given surface.

8. A method as in claim 7 in which said given temperature is in a range between 800 C. and 970 C.

9. .A method as in claim 8 in which said given temperature is less than 840 C. and said cooling step is performed at a rate of approximately 1 C. per minute for reducing said given temperature to a final temperature of between 790 C. and 800 C.

10. A method of producing a selective liquid phase epitaxial growth of semiconductor material with respect to a given surface of a substrate of said semiconductor material, comprising the steps of:

(a) preparing a molten solution of given temperature containing said semiconductor material,

(b) coating said given surface with a masking material that will resist interaction with said-molten solution,

(c) etching windows in said masking material at selected areas of said given surface where growth is to be performed,

(d) supporting said substrate above said solution by a holder of good thermal conduction chracteristics so that the body of the holder is between the substrate and solution, said substrate being heated to a temperature in the order of said given temperature,

(e) immersing said substrate into said solution so that said holder enters first and said substrate enters second with the solution flowing substantially at once over the masked given surface upon immersion, and

(f) cooling said solution so that the contained semiconductor material precipitates out and becomes deposited at said selected areas.

11. A method as in claim 10 wherein said substrate enters said solution with said given surf-ace in an alignment that is within a few degrees of parallel to the surface of said solution.

12. A method as in claim 10 in which the body of said holder is in the shape of a plate and said substrate is secured to the top surface of said plate with said given surface free. a

13. A method as in claim 10 in which said substrate is composed ofthe same semiconductor material as is contained in said solution.

14. A method as in claim 13 in which said given temperature approximately corresponds to a saturated condition of said semiconductor material in said solution, and said substrate'is heated to a temperature slightly higher than said. given temperature so that at said selected areas portions of the substrate are dissolved upon first becoming immersed in said solution prior to cooling.

15. A method as in claim 14 in which said solution in cludes a mixture of GaAs, Ga and an impurity and said substrate is a single crystal GaAs wafer having the III Ga face as said given surface.

16. A method as in claim 15 in which said given temperature is between 800 C. and 840 C. and said cooling step is performed at a rate of approximately 1 C. per minute for reducing said given temperature to a final temperature of between 790 C. and 800 C.

References Cited UNITED STATES PATENTS 3,158,512 11/1964 Nelson et al. 148--1.5 3,463,680 8/1969 Melngailis et a1. 1481.6

RICHARD O. DEAN, Primary Examiner Cl. X.R. 

